1. Field of the Invention
The invention relates to methods of manufacture of light emitting devices with phosphor wavelength conversion. More particularly the invention concerns a method of applying a phosphor material to a light emitting diode (LED).
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being nearly white in color.
An example of the manufacture of a gallium nitride (GaN) based white LED is shown schematically in FIGS. 1a to 1d. As shown in FIG. 1a an LED wafer 10 comprises a wafer (substrate) 12, typically sapphire, that has one or more layers of n-type gallium nitride 14 and p-type gallium nitride 16 materials epitaxially grown on its surface and configured to define a plurality of LED chips 18. Each LED chip 18 has respective n-type and p-type electrode pads 20, 22 on its upper surface for providing electrical connection to the chip. As is known in other device structures one or both electrode pads 20, 22 can be provided on the substrate side of the LED. To fabricate a white LED the LED wafer is 10 is divided (diced) into individual LED chips 18 by for example laser scribing and breaking or by mechanical sawing using a diamond saw (FIG. 1b). Each LED chip 18 is then individually mounted in a package (housing) 24 or mounted to a lead frame (FIG. 1c).
The package 24, which can for example comprise a low temperature co-fired ceramic (LTCC) or high temperature polymer, comprises upper and lower body parts 26, 28. The upper body part 26 defines a recess 30, often circular in shape, which is configured to receive one or more LED chips 18. The package 24 further comprises electrical connectors 32, 34 that also define corresponding electrode contact pads 36, 38 on the floor of the recess 30. Using adhesive or soldering the LED chip 18 is mounted to the floor of the recess 30. The LED chip's electrode pads 20, 22 are electrically connected to corresponding electrode contact pads 36, 38 on the floor of the package using bond wires 40, 42. The recess 30 is then completely filled with a transparent polymer material 44, typically a silicone, which is loaded with the powdered phosphor material(s) such that the entire surface of the LED chip is covered by the phosphor/polymer mixture. To enhance the emission brightness of the device the inner walls of the recess 30 are often inclined and light reflective. Optionally, a lens (not shown), whose dimensions correspond to the dimensions of the recess, is mounted on the package to focus or otherwise direct the light emission of the device.
A drawback with such a method of manufacture is cost, since phosphor has to be applied to the LED chip on an individual basis. In addition the color hue of light generated by the device, or in the case of a white light emitting device the correlated color temperature (CCT), can vary significantly between devices that are supposed to be nominally the same. The problem of color/CCT variation is compounded by the fact that the human eye is extremely sensitive to subtle changes in color hue especially in the white color range. As is known, the correlated color temperature (CCT) of a white light source is determined by comparing its hue with a theoretical, heated black-body radiator. CCT is specified in Kelvin (K) and corresponds to the temperature of the black-body radiator which radiates the same hue of white light as the light source. The CCT of a white LED is generally determined by the phosphor composition, the quantity of phosphor incorporated in the device and its actual location/distribution.
As well as the variation between devices in the color/correlated color temperature (CCT) of emitted light, it is found that the color/CCT can vary across the light emitting surface of the device. The color/CCT depends on the thickness of phosphor/polymer and the distance (i.e. path length) that light travels from the LED chip through the phosphor/polymer encapsulation before being emitted from the device. As shown in FIG. 2, light 46 which is emitted substantially on axis will have traveled a shorter path length 48 within the phosphor/polymer encapsulation than light 50 emitted off axis towards the periphery of the device in which the path length 52 is correspondingly longer. As a result the light 46 emitted substantially on axis will have a higher proportion of blue light compared to yellow (phosphor generated light) and will appear to be blue-white in color. Conversely light 50 emitted off axis towards the periphery of the recess will have a correspondingly higher proportion of yellow light emitted by the phosphor and will appear yellow-white in color. For general lighting applications, where for example a diffuser is used, this variation in color is not a problem as the lit object itself will also increase illumination color uniformity.
Furthermore, in applications in which the device includes further optical components, in particular a lens, to focus the output light, such color/CCT variation can become a significant problem. For example for a white LED which includes a lens, the focused light spot will have a blue-white core with a pronounced yellow-white periphery.
In addition to the problem of non-uniformity in emitted color/CCT due to the variation in path length through the phosphor/polymer encapsulation, it is found that the phosphor material(s) can accumulate unevenly during curing of the liquid polymer resulting in a non-uniform distribution of the phosphor material(s) over the LED chip and in particular on the edges of the LED chip, which will also emit light, where there may be little or no phosphor material(s). As illustrated in FIG. 2 the phosphor material can accumulate on the bond wires 54, on the upper surface 56 of the LED chip, on the floor 58 of the recess and on the inclined light reflective walls 60 of the package. To overcome this problem a greater quantity of phosphor material is often used though this will result in a corresponding decrease in emitted light intensity. The inventor has appreciated that the variation in color hue can additionally depend on factors including:                variations in bonding wire shape and location which can affect the wetting of the phosphor        adhesive bleed out which can affect the wetting of the phosphor        variations in emission direction of the LED chip        variations in the reflector characteristic        variations or aging in the phosphor/silicone blend        wavelength emission distribution of LED chips.        
It is believed that all of these factors can affect the color hue/CCT of light generated by a light emitting device with phosphor wavelength conversion.
US 2006/0097621 teaches a method of manufacturing a white LED comprising dispensing droplets of a high viscosity liquid phosphor paste on an upper surface of the LED such that the phosphor paste is applied onto the upper surface and side surfaces of the LED and then curing the phosphor paste. The phosphor paste comprises a phosphor powder mixed with a transparent polymer resin and has a viscosity of 500˜10,000 cps. The volume of the phosphor paste droplet and viscosity of the phosphor paste are selected such that the phosphor paste covers the upper surface and side surfaces of the LED and allows the phosphor paste to be uniformly applied to the side surfaces as well as the upper surface. After application of the phosphor paste the polymer resin is cured and the LED chip is connected to the package using bond wires. Finally the package is filled with a transparent polymer material to protect the bond wires.
As taught in our co-pending United States patent application publication No. US 2009/0134414, published Oct. 22, 2009, a method of fabricating a light emitting device comprises: heating a light emitting diode chip package assembly to a pre-selected temperature and dispensing a pre-selected volume of a mixture of at least one phosphor and a light transmissive thermosetting material (silicone, epoxy) on a surface of the LED chip. The pre-selected volume and temperature are selected such that the phosphor/material mixture flows over the entire light emitting surface of the LED chip before curing. In an alternative method, using a light transmissive UV curable material such as an epoxy, the phosphor/material mixture is irradiated with UV radiation after a pre-selected time to cure the material. The pre-selected volume and pre-selected time are selected such that the phosphor/material mixture flows over at least the light emitting surface of the LED chip before curing.
To alleviate the problem of color variation in light emitting devices with phosphor wavelength conversion, in particular white LEDs, are categorized post-production using a system of “bin out” or “binning” In binning, each LED is operated and the actual color of its emitted light measured. The LED is then categorized or binned according to the actual color of light the device generates rather than according to the target CCT with which it was produced. A disadvantage of binning is increased production costs and a low yield rate as often only two out of the nine bins are acceptable for an intended application resulting in supply chain challenges for white LED suppliers and customers.
Various methods of applying the phosphor to the LED chip have been proposed in an effort to improve coating uniformity and color hue and CCT consistency. For example, US 2008/0076198 describe a method of manufacturing an LED package comprising: forming a resin mold encapsulating an LED chip and then forming a phosphor thin film on a surface of the resin mold by spray coating a phosphor-containing material on the surface of the resin mold.
U.S. Pat. No. 7,344,952 describe a process for manufacturing LEDs that utilize phosphor wavelength conversion. LED dies (chips) are tested for CCT and binned according to their color emission. The LEDs in a single bin are mounted on a single submount (substrate) to form an array of LEDs. Various thin sheets of a flexible encapsulant (e.g. silicone) containing one or more phosphors are preformed, where each sheet has different color conversion properties. An appropriate sheet is place over an array of LEDs on a submount, and the LEDs are energized. The CCT of the emitted light is measured. If the CCT is acceptable, the phosphor sheet is permanently laminated onto the LEDs and submount. The LEDs in the array are separated into individual devices. By selecting a different phosphor sheet for each bin of LEDs, the resulting CCT is more consistent across the bins. Although such a process can produce LEDs with a more consistent CCT both the LED dies and phosphor sheet need to be binned and this can make the process expensive.
US 2005/0274967 teaches producing a sheet of wavelength converting (phosphor containing) material and then dividing the sheet into individual caps or elements. The light conversion characteristic of each cap is then measured and the caps binned according to their characteristic. The light emission characteristic of each packaged LED is measured and an appropriate cap bonded to the LED to produce a desired CCT of emitted light. Although this process can produce LEDs with a desired CCT, binning of LEDs and phosphor caps is required and this can make the process time consuming and expensive.
U.S. Pat. No. 7,049,159 describe forming a luminescent layer on light emitting devices that are mounted on a substrate. The method comprises positioning a stencil on a substrate such that the light emitting devices are located within a respective opening of the stencil, depositing a composition including the luminescent material in the opening, removing the stencil and curing the composition to a solid state.
US 2006/0284207 teach applying the phosphor material during formation of the LED package. LED chips are electrically connected to pattern electrodes on a substrate such as a PCB or ceramic substrate. Next an encapsulant, epoxy molding compound (EMC), containing the phosphor material is formed on each LED chip by transfer (injection) molding. After curing the encapsulant is cut around the periphery of the chip and a layer of a highly reflective metal formed on the periphery of the encapsulant by electrolysis, electro-plating or sputtering. The reflective layer defines the side wall of the packaged LED. Finally, the substrate is cut horizontally and vertically into individual LED packages.
In our co-pending United States patent application publication No. US 2009/0117672 A1, published May 7, 2009, a method of fabricating a light emitting device having a specific target color of emitted light is described. The method comprises: depositing a pre-selected quantity of the at least one phosphor material on a light emitting surface of the light emitting diode; operating the light emitting diode; measuring the color of light emitted by the device; comparing the measured color with the specific target color; and depositing and/or removing phosphor material to attain the desired target color.
A need exists still for a method of manufacturing light emitting devices with phosphor wavelength conversion that can produce devices with a more consistent color/CCT less expensively than the prior art solutions.